The present disclosure relates to image forming apparatuses and particularly relates to control of the application of bias to a transfer roller.
In recent electrophotographic image forming apparatuses, a toner image formed on the surface of a photosensitive drum is transferred to a recording medium by a transfer roller. In this transfer, a transfer bias applying section applies to the transfer roller a transfer bias of reverse polarity to the charge of toner forming the toner image to be transferred. If the transfer roller is an ion-conductive transfer roller, continued application of the above transfer bias from the transfer bias applying section will cause ionic polarization inside the transfer roller to impair the electric conduction. To cope with this, for example, in a known image forming apparatus, a reverse bias of the same polarity as toner charge is applied from its transfer bias applying section to its transfer roller with the timing of completion of image formation and the timing of an interval between recording paper sheets being successively conveyed, thereby reducing ionic polarization and thus preventing the deterioration (resistance rise) of the transfer roller.
However, if, in the case of using an ion-conductive roller as the transfer roller, a reverse bias is applied thereto in an interval between recording paper sheets as described above, ionic polarization can be avoided to prevent the deterioration of the transfer roller but a history of charge due to the application of the reverse bias remains on the surface of the photosensitive drum. As a result, the toner image formed on the surface of the photosensitive drum causes concentration differences between various portions thereof. Therefore, in the case of using an ion-conductive roller as the transfer roller, a normal bias has had to be continuously applied to the transfer roller for all the intervals between recording paper sheets during continuous printing. To reduce the above ionic polarization and prevent the above deterioration of the transfer roller, it has been necessary to secure enough time for post-aging after the completion of printing.